1. Field of the Invention
The present invention relates to magnetic materials, more particularly to a resin magnet, a magnetic toner, magnetic coating material, and a magnetic recording material.
2. Discussion of the Background
A variety of magnetic materials comprising magnetic powder dispersed in an organic matrix is recently known. Such magnetic materials are embodied typically as resin magnets, magnetic toners and magnetic recording materials.
These products will be reviewed below individually.
In the field of magnets, in order to improve moldability and fragility of conventional sintered magnets, there are used resin magnets prepared by kneading a magnetic powder and a resin. Resin magnets are divided into rubber magnets and plastic magnets, depending on the type of the resin used. However, both of rubber magnets and plastic magnets have the following problems attributable to the necessity of filling magnetic powder uniformly and compactly in an organic polymer matrix, i.e., (i) Density of a magnetic powder is lowered by the addition of a resin; (ii) Because of incoordinate wettability of a magnetic powder and a resin, dispersibility is poor, and viscosity rises during molding to deteriorate fluidity while causing nonuniformity; and (iii) Magnetic properties are deteriorated. Thus, the improvements over these difficulties have been desired.
For the development of static or magnetic latent images, which is one of the recording techniques, there are avilable a dry developing method and a wet developing method. Magnetic toner used in the dry developing method is an ultra-fine powder composed of fine magnetic particles, electric charge regulator particles and other necessary particles for providing various properties, such as particles of a fluidity improver, dispersed in a resin such as polyethylene or styrene-acrylic resin. There is also known a toner in which a colorant is directly attached to the surface of the particles of a magnetic powder, such as disclosed in Japanese Patent Application Kokai (Laid-Open) No. 60-26954.
The important properties to be possessed by a toner are, e.g., fixing properties, anti-block properties, developing properties, transfer properties, anti-offset properties and cleanabiity. In the case of a magnetic toner, the dispersed state of a magnetic powder in the binder affects the magnetic properties of the whole toner.
A magnetic recording material such as a magnetic disc or a magnetic tape is usually produced by kneading a magnetic powder such as a ferrite powder, an organic binder and a solvent to prepare a magnetic coating material and coating it on a substrate such as a resin sheet or film. In this operation, dispersibility of the magnetic powder in the organic binder greatly affects the magnetic properties or wear resistance of the magnetic recording material. Since, however, a magnetic powder is an inorganic matter having a large surface energy, it has poor affinity for a resin having a small surface energy and is therefore very bad in dispersibility if no treatment has been made.
The problem common to these materials is the necessity to further enhance dispersibility of a magnetic powder in the organic matrix. A solution to this problem is to subject a magnetic powder to a treatment for making the particle surface hydrophobic. Such surface treatment has usually been conducted by using a higher fatty acid or an appropriate coupling agent such as a silane coupling agent or a titanium coupling agent. In case of using a higher fatty acid, the acid does not directly react with the functional groups orientated on the surface of the inorganic matter and therefore no covalent bonds are formed. Further, there usually exists surface water on the surface of an inorganic matter, and a higher fatty acid is orientated on the surface with the interposition of the water layer, so that it tends to fall off. In case of using a conventional coupling agent, it is said that the agent forms covalent bonds with the functional groups on the surface of an inorganic matter, and when a surface treated filler is filled in a resin, the silane type coupling agent serves for improving strength of the resin while the titanium type contributes to bettering the working properties. Surface treatment of a magnetic powder is, in many cases, performed by using a titanium coupling agent, as for instance seen in Japanese Patent Application Kokai No. 60-229306, and the practical effect thereof has been actually confirmed. However, the currently used materials can not meet the performance requirements, and the further improvements on surface treating techniques are required.